Eurytemora affinis (Copepoda) were fed 15N-labeled Rhodomonas salina (Cryptophyta) or 15N-labeled
Nodularia spumigena (Cyanobacteria) in excess under controlled laboratory conditions. Zooplankton collected
from the Baltic Sea were fed natural phytoplankton amended with 15N-labeled N. spumigena. We quantified the
direct incorporation of 15N tracer from N2-fixing N. spumigena (diazotroph nitrogen) and ammonium-utilizing R.
salina into the amino acid nitrogen (AA-N) of zooplankton using complementary gas chromatography–
combustion–isotope ratio mass spectrometry, gas chromatography–mass spectrometry, and elemental analysis–
isotope ratio mass spectrometry approaches. Specific and mass-specific TN and AA-N incorporation rates of the
15N tracers were calculated for zooplankton. Highest incorporation of 15N was found in field zooplankton relying
on N. spumigena and in E. affinis relying on R. salina. Lowest incorporation was found in E. affinis relying on N.
spumigena. Decreasing specific and mass-specific rates during field experiments possibly were due to food
shortage, whereas decreasing rates in E. affinis grazing on R. salina were more likely due to satiation. Specific and
mass-specific rates were consistently low in E. affinis when exposed to N. spumigena, suggesting that these animals
were reluctant to feed on N. spumigena. Essential isoleucine received most of the diazotroph nitrogen in field
zooplankton, while nonessential amino acids received most 15N tracer in E. affinis. N. spumigena was clearly an
important amino acid nitrogen source for Baltic Sea zooplankton